Helicopters currently provide the only practical means for vertical transport of cargo payloads and passengers. Alternatively, so called “long line” pick up techniques behind fixed-wing aircraft have been employed but only small amounts of weight may be picked up by any size aircraft. Although conventional helicopters show unequaled flight characteristics and fulfill the need of specific markets, their performance is limited and shows important shortcomings compared to fixed-wing aircraft. Helicopters operate in a very complex aerodynamic and structural environment. As a consequence, conventional helicopter configurations have the following limitations.
For a conventional helicopter, approximately 75% of its power requirement is from the power required to push air downward to create an upward lift, called induced power. The power requirement reduces for larger diameter rotors, as shown in the momentum equation used for conceptual sizing of helicopters where FM (factor of merit) is a correction factor with values between 0.7 and 0.85, ρ is the air density, and A is the area covered by the rotor (πr2). In order to increase the thrust output of the rotor for a given power, the rotor size has to be increased. However, it has been seen that conventional helicopters have limited rotor size due to its complex aerodynamics and dynamic loading, and the increase in weight does not counterbalance the increase in efficiency.
                              P          ind                =                              1                          F              ⁢                                                          ⁢              M                                ⁢                                                    T                3                                            2                ⁢                ρ                ⁢                                                                  ⁢                A                                                                        Equation        ⁢                                  ⁢        1            
Helicopters have limited forward velocity in forward flight. A part of the rotor is operated with an increase in relative airspeed, also referred to as air velocity (advancing side), while the other half of the rotor sees a reduction in relative airspeed (retreating side). In the latter case, a part of the rotor blade operates in a “reverse flow region” where the wind originates from the trailing edge. On the advancing side, the increase in velocity near the rotor tip can lead to shock waves which lead to a reduction in efficiency and high noise levels. The cyclic variation in velocity combined with the requirement that the blades must generate approximately the same lift can lead to dynamic stalls, which also reduces the efficiency in forward flight. The parasitic drag of the fuselage (including the rotor hub) also contributes to the high-power requirement of helicopters at high speed.
The empty weight fraction is rarely below 0.45. The empty weight fraction is the ratio of the empty weight (vehicle weight with no payload, crew or fuel) to gross weight (maximum weight of vehicle at takeoff, with crew, payload and fuel). This ratio shows that a conventional helicopter has limited payload capacity due to the weight of the helicopter hardware required to perform the given task. Given the high fuel burn of conventional helicopters, a small cargo can be transported since a large part of the payload must be reserved for the fuel.
The possibility to perform vertical pickup and delivery of payloads using long tethers towed by fixed-wing airplanes has been proposed as early as 1931 by Beauford in U.S. Pat. No. 1,829,474. The early techniques assumed the use of a single aircraft with a long tether. A single airplane lowers a long tether and maintains a circular flight path. Under specific flight conditions, the tip of the tether becomes the apex of an inverted cone, allowing loading and unloading a payload from an airborne aircraft. A similar method was proposed in the late 1930's by Smith in U.S. Pat. No. 2,151,395 with the additional teaching that the tether could be reeled in from both the aircraft and the payload. The technique was shown experimentally by a missionary pilot, who used this technique to deliver small payloads in remote regions in South America. However, the performance of that system was quite limited since only one fixed-wing aircraft and tether was used.
The main motivation behind the concept of using fixed-wing aircraft to lift payloads in the 1940s was to achieve a higher payload lift capability than rotary wing aircraft, who were in their early days. The interest for a VTOL concept using tethered aircraft is not limited to a few individuals. Large corporations such as Lockheed Martin, in U.S. Pat. No. 4,416,436, and Northrop Grumman, in U.S. Pat. No. 5,722,618, also showed interest for this concept. Lockheed Martin (Lockheed-Georgia Company at the time) and Mississippi State University performed flight tests in 1983 using a Cessna Agwagon and a Boeing Stearman for the fixed-wing aircraft with positive results [Wilson 1983]. However, it was highlighted that the complexity to attach the tethers and the limitations to use conventional, manned aircraft can limit the applicability of the concept. Most of the novel concepts using tethered airplane to lift a payload show the following characteristics:
Use of conventional, manned airplane: Since manned aircraft are used, the airplanes must takeoff from a regular runway and then connect the tethers to the payload while they maintain a circular flight path. Therefore, the concept is not truly VTOL, but only the payload can lift vertically. Manned airplanes are also much heavier given an available power. The source of energy (usually jet fuel or avgas) is located in the airplane wings, onboard the airplanes. Such architecture reduces the thrust-to-weight ratio of the airplanes and limit their maneuverability. Therefore, the airplanes must take-off from a conventional runway and have limited turn radius. None of the configuration considers the use of electric power in a hybrid configuration to improve the overall efficiency of the system and increase its autonomy.
VTOL of the Payload, not the Airplanes: All the vehicle concepts presented in the literature review can perform VTOL of a payload, but no configuration allows for truly VTOL vehicle, where both the aircraft and its payload can take-off vertically.
Limited (Circular) Flight Path: The control methods previously proposed allows only for a circular flight path or a transition between hover (circular flight path) and high-speed flight where the aircraft are flying side by side. Conventional helicopters have six main control degrees of freedom: cyclic control (2), collective, tail rotor pitch, and two pseudo-controls (tilt and bank of the vehicle) provided that the rotor speed is constant. For a given flight condition, such as forward flight at a constant speed, there is only one setting of the controls (trim) that can be used to maintain that flight condition. For example, vertical motion is obtained by increasing the main rotor collective. More complex motions, such as a translation of the vehicle is obtained first with a tilt of the hub plane (initiated with a cyclic control input) and then with the tilt or pitch of the whole vehicle. Therefore, no optimization can be performed on how to trim the vehicle. The reconfigurable rotor VTOL concept shows a much larger number of control degrees of freedom since a large number of flight paths can lead to the same airspeed of the system. A reduction in power requirement throughout the flight envelope (Any condition that the aircraft can fly in (payload weight, altitude, airspeed, air density . . . ). A reduction in power requirement would lead be a more efficient system with more payload capacity, higher endurance and range, faster system airspeed, and reduced operating cost. The use of circular flight paths can be seen as the most obvious choice based on the paradigm of conventional helicopter rotors. Previous research on tethered payload towed by circling aircraft also used this approach given the simplicity to fly such flight path by manned aircraft. However, in the present situation, much more advanced flight path trajectories and airspeed can be desirable to reduce the power required as a function of the flight speed.
In previous proposed concept ideas, such as taught in U.S. Pat. No. 4,416,436, the transition starts with the fuselage with zero airspeed, and is accelerated to the critical airspeed in approximately one revolution of the aircraft. This method of accelerating the fuselage is not possible when the weight of the fuselage becomes large compared to the weight of the aircraft because a lot of kinetic energy coming from the aircraft has to be transmitted to the fuselage in a short amount of time, which would cause a very large power requirement. The transition to the fast flight, where the multiple aircraft fly side-by-side at high speed is a critical aspect of the flight. The transition can only occur when the whole system goes faster than the critical airspeed: the speed at which the aircraft can sustain the weight of the fuselage (stall speed). It can be desirable to use an alternate flight path to accelerate the cargo load gradually.
A significant power loss mechanism in hover is the induced power. For a conventional helicopter, approximately 75% of its power requirement is from the power required to push air downward to create an upward lift, called induced power. The power requirement reduces for larger diameter rotors, as shown in the momentum equation used for conceptual sizing of helicopters:
                              P          ind                =                              1                          F              ⁢                                                          ⁢              M                                ⁢                                                    T                3                                            2                ⁢                ρ                ⁢                                                                  ⁢                A                                                                        Equation        ⁢                                  ⁢        2            
Where FM (factor of merit) is a correction factor with values between 0.7 and 0.85, ρ is the air density, and A is the area covered by the rotor (πr2).
The forward flight phase is defined as a flight condition in which the fuselage airspeed is non-zero and the tethered UAV are still flying along a periodic flight path with respect to the fuselage reference location. Two main problems arise in this flight phase: wake interaction between the aircraft in forward flight, which can be minimized with unconventional flight paths; and increase in tethered UAV and tether losses which can be minimized by performing load transfer between the UAV. All concepts assume that the tethered UAV cannot fly along circular flight path while moving the fuselage. They assume that the system is only in hover or in high-speed flight, but it cannot operate in-between. Tethered aircraft concepts to lift payloads traditionally do not operate in the “slow flight regime”, where the payload velocity is too slow to allow the tethered aircraft to fly “side-by-side” as if they were towing the fuselage. They either perform circular flight path (in hover) followed by a rapid transition to a “linear flight path”, where the aircraft fly side by side. In other words, the system cannot fly “slow”.
Undesirable wake interaction is obtained when one tethered UAV encounter or fly near the wake from another UAV or its own wake after one rotor revolution. In forward flight, for circular flight path, the tethered UAV encounter the wake from the other UAV which increases the power requirement.
In some applications, it can be desirable to limit the motion of the fuselage. Among the configurations, it can be appropriate to be as close to zero motion of the fuselage. Also, dynamic motion can lead to unstable behavior of the system. Consequently, it is important to include means to control the fuselage motion by including thrusters on the fuselage.
Complex flight paths can lead to non-constant electrical power requirement. This is due to the non-constant velocity, and requirement in thrust of each aircraft. The power required for a non-limiting forward flight case example is shown in FIG. 29. It is therefore desirable to limit the drawbacks of the non-constant required power.
Accordingly, there is a need for an improved tethered winged structure for lifting and transporting cargo that is more efficient, can be easily installed and economically manufactured.